1. Field of the Invention
The present invention relates to a method for facilitating discrimination and identification of seismic or microseismic events resulting from seismic monitoring of an underground zone under development.
The method according to the invention is applicable in reservoir zones or in underground cavities under development generally used for either extracting fluids or for injecting fluids therein.
2. Description of the Prior Art
Locating points in an underground zone, either a reservoir zone or a cavity, where microseismic events, linked with an activity resulting in a change in the stress field, is of great interest for good development of the zone, either the production of fluids extracted from a reservoir through one or more wells, or an injection of fluids into the zone.
The evolution of a hydrocarbon reservoir under production or of geothermal deposits can for example be monitored. In case of enhanced recovery notably, the oil is to be flushed out of the rock by injecting fluids at pressures and temperatures that can be very different from those of the environment. The resulting stress variations can lead to a fracturation of the environment which modifies fluid circulations in the reservoir and whose location is important.
It is also well-known to use underground reservoirs for fluid storage. These may be, for example, storage reservoirs for fluids in the liquid or gas phase where a certain microseismic activity induced by significant flow variations of the gas withdrawn from or injected into these reservoirs can be observed.
The reservoirs may also be reservoir zones or cavities used for waste storage, which have to be monitored in order to respect the environment and to comply with increasingly stringent regulations concerning pollution. An underground reservoir can be used for injecting drilling fluids containing solid particles which regulations forbid dumping thereto after use on drilling sites.
The temperature of the fluids injected is generally very different from the temperature of the environment at the depth where they are injected which, in case of massive injection, results in thermal stresses generating fractures and consequently a certain seismic activity. The injection pressure of these muds can also come into play and create stresses, thus leading great changes in the environment.
The seismic activity induced by the effects of the pressure or of the temperature can for example reveal the formation of fractures or stresses on previously existing fractures. They contribute to modifying the flow paths of the fluids in the environment, or they create paths allowing escape of the fluids out of the reservoir, containment breaks with possible pollution of the neighbouring zones, notably of an aquifer developed for potable water supply, which it is essential to detect.
Monitoring of reservoirs used for nuclear waste storage in order to prevent the injection operations leading to local temperature rises due to storage from causing breaks in the containment layers is also of great importance.
Although the mechanical origins of a microseismic activity are quite clear, analysis of the observed phenomena remains delicate in most cases because of the lack of any means allocated for acquisition of the results, the various scales on which the phenomena are observed, limited knowledge of the site investigated, etc. The time factor is also very important. Some events shortly follow the trigger phenomenon. It is for example the case for hydraulic fracturing where location of the events allows mapping all or part of the created fracture. Others produce deferred effects, notably in the case of massive production or of massive fluid transfer from a zone of the reservoir to another, with a range of influence that is often greater than the dimensions of the reservoir.
With microseismic monitoring, also referred to as passive seismic, the operator's aim is eventually to interpret very rapidly the data in connection with conventional production data (pressure, flow rate, temperature, etc.) so as to be able to take account of the mechanical response of the site in the development protocol in order to preserve the productivity of the well(s) or of the site. The observed microseismic activity observed can be associated with a mechanical degradation of the environment that can generate the occurrence of solids, with the opening of fractures that can communicate the reservoir with an aquifer, or with other phenomena of thermo-poro-mechanical origin whose consequences can induce a degradation of the well performances or even to damages.
Detection and location of seismic or microseismic events in a reservoir can be achieved, as it is well-known, by lowering into a well, at the end of a cable, a sonde containing a triaxial seismic receiver that is pressed against the wall of a well. Comparison of the signals picked up by the various pickups of the seismic receiver in the sonde (analysis of the polarization of the waves received) allows determination of the direction in which the seismic event has occurred, provided that the propagation environment is relatively homogeneous, and even to locate it when the signals received contain a succession of P type and S type waves.
For implementation of such a method, the well has to be cleared long enough for the sonde to be lowered to the desired depth, which is not compatible with long-term monitoring.
French Patents 2,593,292; 2,681,373; 2,685,139; and 2,703,470 notably describe various techniques for monitoring the evolution in time of underground reservoirs, comprising using seismic or other pickups permanently installed in one or more wells, without disturbing the various operations (production, injection, various operations carried out by means of these wells). Permanent installation of these pickups in wells allows seismic monitoring of a reservoir in order to detect various phenomena connected with the development thereof.
Permanent seismic pickups are for example installed outside a casing that is lowered into the well. They are embedded in the cement that is normally injected into the annular space between the casing and the well, which provides proper coupling with the surrounding formations.
The seismic pickups can also be fastened outside a production string installed in a well during completion operations. They are associated with mobile device suited to press them against the casing of the well, and with decoupling device for filtering the vibrations propagating along the production string.
Recording of the microseismic activity, also referred to as passive seismic or microseismic monitoring, is enhanced by using permanent well pickups when phenomena are located at great depths (from several hundred meters onwards).
French Patents 2,703,457; 2,703,470; and 2,728,973 describe long-term repetitive monitoring methods in reservoirs by application of elastic waves to a formation and acquisition of response signals reflected by the formation, by the permanent installation of emission and reception in wells or in the neighbourhood of the ground surface. Differential processings are applied to acquisitions achieved under identical conditions.
French Patents 2,688,896; and 2,689,647 notably also describe electronic acquisition and transmission systems specially designed to collect the signals from permanent pickups installed in wells outside casings or production strings, and to transmit them to a surface recording and control equipment during long-lasting repetitive monitoring or seismic surveys.
In the case of a well dedicated to microseismic monitoring, the receivers and acquisition systems that are currently available work under good conditions. It is possible to parameterize them so that they record only significant signals for characterization of the site, such signals being referred to as E type signals.
On the other hand, event identification is more difficult if the monitoring system is placed in a well under development because events induced by completion operations, referred to as C type events hereafter, are also recorded. These events are generated by injection stops and resumptions, opening or closure of one of the completion elements (valve, packer, etc.) that can be at any depth in the well or at the surface (on the wellhead) and even at the level of the surface installation (lines, various devices). Some of these actions, such as communicating the underground zone (reservoir) with the surface network can induce therein, as a result of pressure variations notably, E type events that are often deferred in time and that are desired to be recorded and interpreted. C type events, which can be in large numbers in a relatively short time interval (more than 3400 events within one week for example), harm real-time monitoring of the geomechanical phenomena induced through acquisition of E type events whose number, within the same period, is often relatively low (several ten events for example).
It is therefore very difficult to rapidly discriminate the data files (records of the signals received) corresponding to the E type microseismic events coming from the formations from those corresponding to C type events induced during completion. All (E and C type) events meet the criteria of the commonest digitizing algorithms: detection by exceeding an amplitude or energy threshold and/or detection by exceeding a threshold for a slope ratio representing the slope of the signal in a short time interval divided by the slope of the signal in a longer time interval (triggering on a transient event and not on a signal drift), etc. In order to differentiate between E and C type events, a more suitable algorithm is required, which takes into account the spectrum of the signal, the transit times associated with particular wave reflections during completion, amplitude variations between various wave types, all these operations being much more difficult to control in real time.